Apparatus and method for increasing carbon content of hot directly reduced iron

ABSTRACT

A method of producing a hot, carburized metallized iron product in a generally vertical shaft furnace having an upper reducing zone in which iron oxide reacts with a gaseous reductant, and a lower carbon control and product discharge zone, including: establishing a gravitationally descending iron oxide burden in the furnace; reacting hot gaseous gaseous reductant with the descending burden to form a metallized iron product and a reacted top gas; and injecting a hydrocarbon gas mixture consisting of hot reformed reducing gas and cool natural gas to the product discharge section of the shaft furnace; whereby the carbon content of the metallized iron pellet product is controlled by mixing the reformed gas and natural gas in the proper ratio to balance the endothermic and exothermic reactions within the discharge zone of the furnace. Apparatus for carrying out the method includes means for controlling the respective amounts of gas introduced to the product discharge zone from the reformer and the source of natural gas.

BACKGROUND OF THE INVENTION

This invention relates generally to the direct reduction of iron oxidematerials to produce metallized iron in solid state such as hotmetallized pellets or hot sponge iron in a direct reduction shaftfurnace. "Metallized" as used throughout this specification and theappended claims means substantially reduced to the metallic state i.e.always in excess of 75% metal, and usually in excess of 85% metal in theproduct. Such metallized pellets or sponge iron are well suited as feedmaterials to steel making furnaces such as an electric arc furnace.

Clark et al, U.S. Pat. No. 4,054,444 teaches means for controlling thecarbon content of direct reduced iron pellets when discharged cold froma direct reduction shaft furnace. The gas injected in the Clark et alpatent is methane, natural gas, or heavy hydrocarbon gas, to whichoptionally can be added clean spent top gas from the direct reductionfurnace. The gas is injected into the buffer zone, which is the zonebetween the reduction zone and cooling zone in the furnace. One of thefunctions of the Clark et al invention is to precool the burden beforeit reaches the cooling zone to reduce the required cooling within thecooling zone. The present invention requires the avoidance of thiscooling effect.

Currently, there are three known methods for increasing the carboncontent of direct reduced iron product, all of which are implemented incommercial operation. These three methods are:

(1) lowering the reducing gas temperature at the furnace bustle;

(2) increasing the methane or other hydrocarbon content of the reducinggas to the bustle by adding natural gas; and

(3) injecting natural gas into the lower, or discharge section, of thefurnace.

Each of these methods increases the carbon content of the direct reducediron product, but each method also has limitations in normal furnaceoperation.

Lowering the reducing (bustle) gas temperature has proven to increasethe carbon content in the product in operating direct reduction plantsaround the world, however, the plant production (output) also suffers areduction, due to slower reducing reactions. This loss of productioncapacity with lower reducing gas temperatures has been verified by plantoperating history over many years.

Increasing the hydrocarbon content of the reducing gas by adding naturalgas to enrich the reducing gas at the bustle has been tried to raise thecarbon content of the product. The added hydrocarbon in the reducing gascracks at high furnace temperatures, adding more carbon to the product.

The cracking of these hydrocarbons produces carbon which is integratedinto the product, and hydrogen which flows upwardly through the shaftfurnace where it acts as additional reductant gas for reducing the ironoxide to metallized iron (or direct reduced iron) in the upper reductionzone of the shaft furnace. The amount of hydrocarbon that can be addedto the furnace is limited because the cracking of hydrocarbons is anendothermic reaction. An overabundance of hydrocarbons in the reducinggas, when cracked to form (C) plus hydrogen (H₂), causes a cooling trendin the shaft furnace. The resulting reduction in burden temperaturecauses a slower reduction reaction between the reducing gas and the ironoxide, and, ultimately, lower production. In addition, in a hotdischarge/hot briquetting (HD/HB) direct reduction plant, the addedcooling adversely affects the ability of the metallized iron product tobe briquetted, a situation which must always be avoided.

Injection of natural gas into the lower cone (cooling and discharge)region of the shaft furnace is also a proven method of adding carbon tothe product in direct reduction plants. In a cold product dischargeplant, this is an excellent and economic method of adding carbon to theproduct. It is limited only by the amount of cooling that can betolerated in the upper (reducing) section of the shaft furnace withoutsignificantly reducing the furnace output or product quality. The usualdesired level of carbon addition to the product can be easily achievedwithout reaching the point of over-cooling the burden, since it isdesirable to discharge the product at near ambient temperatures. InHD/HB plants, an added product specification must be met in addition toproduction rate and product quality; the product must be sufficientlyhot on discharge to be compacted into briquets. It is this productrequirement that severely limits the amount of natural gas that can beinjected into the lower portion of the hot discharge furnace. Theendothermic reaction of cracking the natural gas can cool the burdenbelow the minimum temperature for good briquetting. The three methodsdescribed above all have the same limitation of temperature. Thereduction temperature in the furnace must be maintained above at least760° C. if production is to be maintained. In the case of an HD/HBfurnace operation, a high discharge temperature (above about 700° C.)must also be maintained to insure good briquetting. This finaltemperature requirement for hot discharge plants severely hinders theeffectiveness of these three methods to deposit the desired amount ofcarbon in the product.

The problem is twofold: first, to add carbon to the product, and second,to avoid contributing any significant endothermic load to the furnaceburden. The present invention overcomes both of these problems by makinga controlled addition of hot reformed gas enriched with natural gas tothe furnace discharge zone.

The accomplishment of both of these objectives rests in the fact thatthe reformed gas/natural gas mixture forms a "balanced" system, from aheat of reaction standpoint. The disadvantage to adding natural gas tothe furnace is the endothermic cracking reactions that cause coolingwithin the furnace. In the reformed gas/natural gas mixture, there is abalancing reaction to the cracking reactions:

    2CO (g)=C (s)+CO.sub.2 (g)

This is the Boudouard reaction. This reaction is possible because of thehigh CO content in the reformed gas. As the temperature begins to fallin the furnace because of the cooling effect from the cracking of thenatural gas, the equilibrium of the Boudouard reaction favors carbondeposition to a greater extent. The deposition of carbon from theBoudouard reaction is an exothermic reaction. Therefore, by mixing thereformed gas and natural gas in the proper ratio, a balancing of theendothermic and exothermic heat loads in the furnace is realized. As thenatural gas cools the burden by cracking, the CO restores the lost heatby decomposing to CO₂ and solid carbon.

The natural gas - reformed gas mixture is injected into the lower coneof the furnace at temperatures at or above the required minimumtemperature to insure good briquetting. This inlet temperature iscontrolled by the amount of cold natural gas used to enrich the hotreformed gas. Since the reformed gas/natural gas mixture to the lowercone is hot, it provides an additional benefit during plant start-up.

The reformed gas/natural gas mixture provides more carbon than theenriched bustle gas method because of the lower temperature in the lowercone region of the furnace. Bustle gas temperatures are sufficientlyhigh to crack the heavy hydrocarbons in the natural gas, but thetemperature is too high for the Boudouard reaction to be carbondepositing. In the lower cone region, the temperatures are lower thanbustle gas temperatures. They are cool enough that the Boudouardreaction favors carbon deposition, while still being warm enough tocrack the hydrocarbons in the natural gas portion of the mixture. It isthis slightly cooler environment in the lower cone region that makesthis method better than simply enriching the bustle gas with naturalgas. With these cooler temperatures there is a double carbon benefit notrealized at bustle temperatures.

Finally, the hot reformed gas/natural gas mixture addition at a mixtureratio where furnace burden cooling does not occur will provide a hotupflowing gas to the reducing zone of the furnace. Whereas the additionof natural gas alone provides a cold gas that flows up the center of thereducing zone from the lower cone region, the reformed gas/natural gasmixture provides a much hotter gas to the furnace center.

In summary, by the invented method, a reformed gas/natural gas mixtureadded to or injected into the lower discharge region or cone of a directreduction furnace provides as much or more carbon content in the productthan natural gas alone. The mixture ratio is controlled to preventburden cooling, and on startup, the process will speed up the burdenheating and initial reduction phase. The invention provides the soughtsynergistic result; more carbon, no cooling.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to provide a methodand apparatus for producing a higher carbon content direct reduced ironproduct in a direct reduction furnace without adversely affectingoverall furnace operation.

It is also an object of this invention to provide means for avoiding anysignificant endothermic reaction with the burden of a direct reductionfurnace.

It is another object of the present invention to provide a method andapparatus for controlling a gas mixture for injection into a directreduction furnace discharge zone which will not adversely affect furnaceoperation.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and other objects of the invention will become betterunderstood by referring to the following detailed description and theappended drawing, in which:

The single figure is a schematic flowsheet showing the operation of andthe apparatus of the subject invention.

DETAILED DESCRIPTION

As shown in the single figure, the invented process utilizes a verticalshaft-type reducing furnace 10 having an upper reducing zone 12 in theupper region of the furnace, a bustle gas introduction zone 14 in thecentral region of the furnace and a carbon control and product dischargeregion 16 in the bottom portion of the furnace. Iron oxide pellets orother materials such as lump ore are charged into the shaft furnace bygravity to form a bed of particulate iron oxide containing material, orburden, within the shaft furnace. Metallized, or reduced, material isremoved from the furnace through outlet 20 at the bottom thereof. Abustle and tuyere system indicated generally at 24 surrounds the shaftfurnace. Hot reducing gas is introduced to the reducing zone through gasports within the bustle gas system. The hot reducing gas flows inwardlyand upwardly through the reducing zone in counterflow relationship tothe gravitational movement of the burden. The reducing gas reacts withthe burden to form a top gas which exits the furnace through gas takeoffpipe 30 at the top of the furnace.

A reformer furnace 40 having fuel fired burners, not shown, and aplurality of indirect heat exchanger catalyst tubes 42, which areexternally heated, only one of which is shown, generates hot reducinggas. The reducing gas flows from the catalyst-containing tubes 42through reformed-gas pipe 44. A portion of the reformed gas passesthrough pipe 46 to bustle and tuyere system 24, a second portion of thereformed gas flows through pipe 48 to a hot venturi 50. Gas pipe 52connects venturi 50 with valve 54, which is in turn connected to thelower cone portion 16 of the furnace by pipe 56.

Natural gas source N is connected to pipe 56 by natural pipe gas 58,which has a metering orifice 60 and flow control valve 62 therein.

The electrical controls for the process include a flow controller 68which receives a signal from hot venturi 50 and controls valve 54, flowcontroller 70 which receives a signal from natural gas metering orifice60 and sends a signal to valve 62. Both flow controllers 68 and 70 areconnected to a ratio station 72, which is a computerized controller.Thermocouple 74, within the bottom of the shaft furnace may be connectedto ratio controller 72, if desired, but is generally provided with anoptical readout for use by the operator. Thermocouple 76 in pipe 56 onthe shaft furnace side of the connection with pipe 58, which connectionis the mixing point of the gases, can also be connected to ratio station72. Gas analyzer 78, in pipe 56 near the shaft furnace, which isconnected to ratio station 72, analyzes the methane composition of thegas in pipe 56.

In operation, process gas from source G, which can be spent top gas fromshaft furnace off-take 30, natural gas, methane, or a mixture thereof,is reformed to substantially CO and H₂. The reformed gas is divided,part passing directly into the the bustle and tuyere system 24 asreducing gas, and a second portion being metered through hot venturi 50which feeds a signal to flow controller 68 that activates hot valve 54to maintain the flow at a specified setpoint. Natural gas from source Nis fed into the system at ambient temperature, and metered throughorifice 60. The metering orifice generates a signal to flow controller70. The flow signal from the reformed gas hot venturi is transmittedfrom flow controller 68 to ratio station 72.

At ratio station 72, the setpoint for the natural gas flow controller 70is computed and transmitted to controller 70 for implementation. By thiscontrol system, a fixed mixture ratio of reformed gas to natural gas ismaintained. Gas analyzer 78 determines the methane (CH₄) content of thegas mixture prior to its injection into the lower cone, and transmitsthis methane reading to ratio station 72 which adjusts the ratio ofnatural gas flow to reformed gas flow to give the desired methanecontent.

Thermocouple 74, located in the product discharge chamber 16 of thefurnace 10, registers the temperature of the burden after it has passedthe gas mixture injection point. If the temperature drops too much uponinjection of the gas mixture, station 72 can either reduce the amount ofnatural gas in the mixture, or can reduce the flow rate of the mixtureinto the furnace. If the temperature in the discharge chamber 16 risestoo high, the natural gas flow can be increased, or the flow rate of thegas mixture can be increased, either of which will bring the temperaturedown to the desired range. Station 72 determines which course should befollowed, i.e., alter the mixture ratio or change the flow rate of themixture, according to the mixture temperature as recorded bythermocouple 76. As the natural gas addition is reduced, thistemperature approaches the hot reformed gas temperature, less theinherent heat losses through the piping. A sharp rise in the temperatureas indicated by thermocouple 74 in the product discharge chamber couldindicate too much CO reaction, in which case the natural gas flow shouldbe increased to prevent localized overheating of the burden. The carboncontent of the metallized iron pellet product is responsive to thehydrocarbon gas content of the mixture determined by mixing theconstituents of the mixture in the proper ratio to balance theendothermic and exothermic reactions within the discharge section of thefurnace.

SUMMARY OF THE ACHIEVEMENTS OF THE OBJECTS OF THE INVENTION

It is clear from the foregoing that the present invention overcomes theproblem of cooling direct reduced iron by endothermically crackingmethane or its equivalent to produce carbon, and by exothermicallydisassociating carbon monoxide to obtain carbon, thus balancing theexothermic and endothermic reactions within the discharge section of theshaft furnace.

What is claimed is:
 1. In a vertical shaft-type reducing furnance havingan upper reducing zone, an intermediate reducing zone and a lower carboncontrol and hot product discharge zone, means for introducing reducinggas intermediate the ends of the shaft furnance, means for removingmetallized product from the bottom thereof, and means for removingreacted top gas from the top of the furnance, the improvementcomprising:a reformer furnance, having a source of process gascommunicating therewith and a hot reformed gas exit; a first conduitcommunicating between the hot gas exit of said reformer and the meansfor introducing reducing gas to the shaft furnace; a second conduitcommunicating with said first conduit and with the carbon control andproduct discharge zone of the shaft furnance; a source of natural gas; athird conduit communicating between said source of natural gas and saidsecond conduit; and means for controlling the respective amounts of gasintroduced to said product discharge zone from said reformer and saidsource of natural gas, and for maintaining the temperature within saidproduct dishcarge zone at least 700° C.
 2. Apparatus according to claim1 wherein said second conduit includes a venturi and a first flowcontrol valve therein, and means for adjusting said first valve inresponse to a signal generated by said venturi.
 3. Apparatus accordingto claim 2 wherein said third conduit includes a metering orifice and asecond flow control valve therein, and means for adjusting said secondvalve in response to a signal generated by said natural gas meteringorifice.
 4. Apparatus according to claim 3 wherein said means foradjusting said first valve and said second valve are flow controllers.5. Apparatus according to claim 4 wherein said flow controllers areconnected to a computer-controlled ratio station.
 6. Apparatus accordingto claim 1 further comprising a thermocouple within said productdischarge section of said shaft furnace between the elevation of saidsecond conduit and said product discharge means.
 7. Apparatus accordingto claim 1 further comprising means in said second conduit for analyzingthe gas contained therein.
 8. Apparatus according to claim 1 furthercomprising means for determining and reporting the gas temperatureimmediately downstream of the intersection of said second conduit andsaid third conduit.
 9. A method for controlling the carbon content ofhot metallized iron pellets produced by counter-current flow of hotreducing gases through a downwardly moving iron oxide burden in asubstantially vertical shaft furnace, said furnance having an upperreducing zone, and a lower product discharge zone, said methodcomprising:introducing particulate iron oxide to the interior of saidfurnace to establish a burden therein; removing a portion of said burdenfrom the bottom of said product discharge zone to establish agravitional descent of said burden; introducing hot reducing gas intosaid reducing zone to react with said descending burden and form areacted top gas; removing reacted top gas from the top of the reducingzone to establish countercurrent flow of reducing gas through saiddownwardly moving burden; introducing at a temperature of at least 700°C. a hydrocarbon gas mixture of hot reformed reducing gas and coolnatural gas to the product discharge section of said shaft furnance;whereby the carbon content of said metallized iron pellet product iscontrolled by varying the hydrocarbon gas input conditions to thefurnace, and cooling of said product is avoided.
 10. A method accordingto claim 9 wherein said hot reducing gas is a methane-containingcatalytically reformed gas.
 11. A method of producing a hot, carburizedmetallized iron product in a generally vertical shaft furnace having anupper reducing zone in which iron oxide reacts with a gaseous reductant,and a lower carbon control and product discharge zone, said methodcomprising:introducing particulate iron oxide to the interior of saidfurnace to establish a burden therein; removing a portion of said burdenfrom said discharge zone to establish gravitational descent of saidburden; introducing a hot gaseous reductant to said descending burden insaid reducing zone to react with said burden to form a metallized ironproduct and a reacted top gas; removing said reacted top gas from thetop of said furnace to establish countercurrent flow of said reducinggas through said descending burden; injecting a hydrocarbon gas mixtureconsisting of hot reformed reducing gas and cool natural gas to theproduct discharge section of said shaft furnace; whereby the carboncontent of said metallized iron pellet product is controlled by mixingthe reformed gas and natural gas in the proper ratio to balance theendothermic and exothermic reactions within the discharge zone of thefurnace.
 12. A method according to claim 11 wherein said hot reducinggas is a methane-containing catalytically reformed gas.